Microscopy With a Quantum Tip

Picture of a scanning probe microscope

With their help, phenomena in the nanoworld become visible and targeted manipulation becomes possible. The heart of a scanning probe microscope is a moveable, suspended tip, which, like the needle on a record player, reacts to small height variations on the surface, and turns these into signals that can be displayed on a computer.

Tübingen researchers have now been able to create this tip, not out of solid material, as in the case of the record player, but out of an ultra-cold, dilute gas of atoms. To do this, they cooled an especially pure gas of rubidium atoms to a temperature less than a millionth of a degree above absolute zero temperature, and stored the atoms in a magnetic trap.

This “quantum tip” can be precisely positioned and enables the probing of nanostructured surfaces. With this method, more accurate measurements of the interactions between atoms and surfaces are possible and further cooling of the probe tip gives rise to a so-called Bose-Einstein condensate, which allows a significant increase in the resolution of the microscope. The work was led by Professor József Fortágh, head of the Nano-Atom-Optics group, and his co-worker Doctor Andreas Günther and Michael Gierling.

The scientists demonstrated the use of their cold-atom scanning probe tip by testing a surface with vertically grown carbon nanotubes. The tip was scanned over the sample using a type of magnetic conveyor belt. The first measurements in the so-called “contact mode” revealed how the tall tubes stripped some atoms out of the atom cloud. These atom losses told the researchers about the location and height of the nanotubes and enabled the imaging of the surface topography.

The microscope also functions in the so-called “dynamical mode”. The researchers again created a Bose-Einstein condensate close to the nanotubes. They then allowed the condensate to oscillate perpendicular to the surface, and observed how the frequency and size of these oscillations changed, depending on the topography of the nanostructured sample. In this way they were able to obtain a well resolved image of the surface. The researchers write that this method has an advantage because no atoms are loss from the cloud. This could be helpful in cases where atoms that are adsorbed on the sample might influence subsequent measurements.

COMPAMED.de; Quelle: Universität Tübingen